Here are three wire feed rollers. The right one came with
the Airco and has grooves for 0.035 (0.9mm) and 0.045 (1.2mm)
wire. I use a lot of 0.030 and 0.025 (0.8 and 0.6mm) so I needed
more rollers. They have long been unavailable.

The middle one was made on my 109 lathe using mild steel.
The woodruff key slot was made with a jig saw; crude but it worked.
I had used a 90-degree V-tool to cut the grooves. That worked,
but did not grip well.

I have also made one for a Clarke welder as to not have one
with a knurled groove (better for aluminum).

The left one is described below.

Here is a closeup of the left one. It was made out of 1 3/8"
stainless steel. The steps to make it are as follows:

Use a 4-jaw chuck and center as best as possible.

Do a very light pass to true the steel if needed. I only
had the exact size available, larger turned down to size would
be better (and can be done on a 3-jaw chuck.)

Center bore for hole size. I bored it 5/16" then used
a very small boring bar to bring out to 3/8".

Use a face cut to true up end.

Position 60-degree V cutter for grooves (threading tool will
work.)

Advance cutter until it just touches the work. Then turn
inward the diameter of the wire. It turns out, on a 60 degree
V, cutting the depth of the wire will make a groove that about
1/3 the wire will sit above the groove.

Use a parting tool and cut to width.

If your V-tool does not come to a complete point, you will
have to do less of a cut. Start with 2/3 of the wire size and
put a wire in the groove to test fit.

This roller works perfectly. Using the 60-degrees makes the
V-groove and the pinch roller form a perfect triangle so pressure
is even on all sides.

Here is a closeup of cutting the woodruff key. It is
easiest to do it on the back side of the hole as you can use
the cross slide to advance for each pass. Here is the steps:

Remount the roller in the chuck and lock the spindle.

I used a 1/4" HSS tool bit cut as a 1/16" parting
tool. Mount in in an adjustable tool mount pointing toward the
end of the roller. If needed grind a steep relief angle in the
tool.. Also, angle the tool a bit toward the slot; that will
provide a relief angle on the front and back edges. This tool
was set steeper than I wanted but it was manually ground and
got a bit wider on the top edge. It should be a few degrees from
parallel with the slot.

Adjust the tool so one edge is even where one side of the
slot will be cut (the slot is wider than the tool.)

Slowly move tool through hole. On each pass advance the cross
slide about 1-2 thousandths. This will take many passes and I
repeated some if it felt rough. When the desired depth is cut,
move tool back out and lower or raise it to match the other side
of the slot.

Repeat advancing cutter as before until the desired depth
is reached.

Reset the cutter to the middle if needed to remove the center.

The parting tools are available. Look for a 1/4" square
tool that has the end ground for parting. Usually they are ground
on one side to leave 1/16" remaining a picture of a unsharpened
blank is shown below:

A broaching tool can be used or made, but this is a cheap
solution that works surprisingly well.